Integrated lightweight card rack

ABSTRACT

A plurality of heat sinks assembled to form a rack in a lightweight housing accommodate printed circuit cards between the heat sinks and circulate cooling oil through the heat sinks to drain the test heat from the boards quickly. Each heat sink uses a porous metal foamed core, configured in zigzag shape, to provide a tortuous path for the fluid flow which extends into lateral fins comprising the side plates of the porous metal foam for receiving the cards or ground plates therefore. These plates are clamped against the fins for close contact to the cooling fluid thereby shortening the heat flow path. Brazed on aluminum top and bottom cover plates make a fluid tight connection with the porous foamed metal. The brazed on top and bottom plates are also configured in the zigzag pattern of the porous foamed metal. Lightweight thin aluminum feelers or plates conform to the zigzag sides of the porous foamed metal and are clamped thereagainst by side covers so that the feelers melt in a brazing process for sealing the side covers in fluid tight core conforming relationship with the porous foamed metal core.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is a new lightweight card rack for printed wiring boardsand method of more efficiently extracting the heat from the boardsimmediately upon testing completion.

2. Prior Art

Prior art heat extractors suffer from three large disadvantages:

1. They are inefficient in removing heat,

2. They do not minimize the length of the heat removal path, and,

3. They are bulky and heavy.

SUMMARY OF THE INVENTION

A plurality of heat sinks assembled to form a rack in a lightweighthousing accommodate the cards between the heat sinks and circulatecooling oil through the heat sinks to drain the test heat from theboards quickly. Each heat sink comprises a porous metal foamed core,configured in zigzag shape, to provide a tortuous path for the fluidflow which extends into external fins comprising the side plates of themetal foam core. The external side plate metal fins receive the cards orground plates therefore. These plates are clamped gains the fins forclose contact to the cooling fluid thereby shortening the heat flowpath. Brazed on aluminum top and bottom cover plates make a fluid tightconnection with the porous metal foamed core.

The brazed on top and bottom plates and the porous metal foamed core areconfigured in the same zigzag patterns imultaneously by a programmedelectric discharge machine. Lightweight thin aluminum feelers shaped toconform to the zigzag sides of the foam are clamped thereagainst by sidecovers so that the feelers melt in a brazing process for sealing theside covers in fluid tight core conforming relationship with the porousmetal foamed core.

After brazing, the side covers have milled uniform slots formed alongtheir exteriors for receiving the boards which are clamped against oneside of the so-formed fins by wedge locking devices.

The heat sinks are terminated in fluid communicating blocks whichreceive fluid communicating bellows so that, e.g., four heat sinks (coldplates) are assembled in spaced relation across a rack or housing bottomplate, preferably of foamed metal covered by aluminum on both sides.Then, four stacks of these cold plates comprise the proper height forreceiving the card or board edges.

The corner blocks on one side are terminated in an input manifold and anoutput manifold for directing cooling oil into and out of the bellowsand foam cores for cooling the cards. Four or five graphite windowframes also having a top and bottom flange surround and hold in positionthe cold plates within the rack which has a rack housing top with foamedmetal core, and lightweight aluminum sides.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a prior art cast metal heat sink having a fluid path alongthe axis thereof;

FIG. 1A is a detailed showing of a portion of FIG. 1;

FIG. 2 is a view in perspective of the heat sink core of the presentinvention showing the fluid porous metal foamed core with a lightweightaluminum top cover and aluminum bottom cover;

FIG. 3 is a view in perspective of the foamed porous metal core with topand bottom covers configured to the zigzag shape of the metal foam;

FIG. 4 is a perspective view of the porous metal core with top cover andbottom cover from a different angle than FIG. 2;

FIG. 5 shows the shape of the top cover in plan;

FIG. 6 shows the shape of the bottom cover in plan;

FIG. 7 shows the thin feeler plates configured to the shape of the corefor juxtaposition therewith from both sides;

FIG. 8 shows a prior art heat sink in perspective;

FIG. 9 shows the left-hand side plate configured the same as the porousmetal foamed core and top and bottom cover plates;

FIG. 10 shows the top and bottom covered porous metal foamed core inposition to mate with the side cover of FIG. 9;

FIG. 10A and FIG. 10B show terminal blocks for the heat sink;

FIG. 11 is a perspective view of the right-hand side cover ready to matewith the core configuration;

FIG. 12 shows a portion of the heat sink after mating and brazing, inmagnified detail;

FIG. 13 is a typical rack housing useful in containing the heat sinksand shown with input and output fluid connections;

FIG. 14 is a perspective view of four of the racks deployed in spacedapart relation on the bottom housing cover and interconnected by bellowswith the inlet and outlet manifold adjacent to the heat sink terminatingblocks to which they will connect;

FIG. 14a is a view in perspective of a fluid inlet manifold connector;

FIG. 14b is a view in perspective of a fluid outlet manifold connector;

FIG. 15 is a perspective view of the lightweight housing and rackassembled together with the top housing top cover or lid being partiallyopened;

FIG. 16 is an exploded view of the structure of FIG. 15 to show furtherparts thereof;

FIG. 17 is an exploded view of the rack, per se, which fits within thehousing;

FIG. 18 is a detailed cross sectional view showing the graphite framebonded to an upper and lower heat sink;

FIG. 20 is a view, in side elevation, of a preferred wedge lockingdevice for clamping the boards against the thin walled fins of the heatsink;

FIG. 21 shows the device of FIG. 20 cut in segments on the bias topermit displacement of one or more segments when tightened;

FIG. 22 is an end view of FIG. 21;

FIG. 23 shows the wedge lock tightened into locking position to clampthe board between adjacent fins; and

FIG. 24 is a top plan view of the wedge lock clamping the ground plateof a board between adjacent fins.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

The invention is an integrated cooled lightweight card rack and housingfor containing printed wiring boards for particularly heavy dutyapplications, which card rack and housing is utilized for testing theprinted wiring board by applying predetermined amounts of, heatgenerated by currents applied to the boards, then rapidly removing theheat; and also permitting vibration and other tests to rigidspecifications.

In FIG. 1, a prior art type heat sink for a rack is shown in perspectivecomprising a relatively heavy metal casting 27 with radiating fins 29and with the inset of FIG. 1A showing typical heat flow resistance pathsR1 and R2. R3, which is not shown, is located along the wiring board tothe edge of a fin 29. The printed wiring board 31, or the metal groundplane or support therefore, is clamped against the fin 29', and the heatflow from the heated printed, circuit must travel up the ground plane31, across a portion of fin 29 (R2), then inwardly to the circulatingfluid, generally located at 33, causing the further heat resistance pathR3 to be added to R2 and R1.

In the formula ##EQU1## R is the total heat resistance, L is the lengthof the heat path, K is a fixed coefficient of thermal conductivity and Ais the area of the printed wiring board to be cooled.

Since K and A are constants, once a card size is selected, then only Lcan be diminished to reduce the heat resistance R. Thus, the presentinvention employs materials and arrangements of the materials todiminish the thermal resistance R. Also, due to the unique method offabrication, the invention provides, among other things, the mostefficient way to remove heat known to date, a lightweight structure, anda readily machinable device which may be assembled using knownprocesses.

In FIG. 2, the basic novel heat sink 41 is shown in perspective having afluid porous metal foamed core 43 which transmits fluid only down thetortuous path extending outwardly into fin 45, and then outwardly intofin 47 and then outwardly into fin 49, while avoiding a direct axialpath through the heat sink 41. The forced fluid flow entering the porousmetal foamed core 43 is usually lightweight oil such a "MOBIL ONE" andit is caused to pick up more heat because it actually flows out into thefins along a path which allows the fluid to pick up more heat because itis not axial but is tortuous with the cards giving up heat to the fins45, 47 and 49, etc. which substantially shortens the path (L) of heatresistance from the heat source (PWB) to the fluid flow. The heat sink41 is of course fluid-tight so that the fluid flow is confined to theporous metal foamed core which extends in zigzag fashion down the lengthof the heat sink 41. Note that the fins 44,47 of the present inventionare staggered, thereby increasing the tortuous path, whereas the priorart fins 29 are not staggered.

In FIG. 3, a perspective view is shown of the configured porous metalfoamed core 43 having a brazed-on aluminum top cover 51 and acorresponding bottom cover 53. Note, that the foamed core 43 and the topand bottom covers are precisely configured the same, which is donesimultaneously.

In FIG. 4, the exposed porous metal foamed core 43 is seen from one endof heat sink 41 with the top and bottom covers 51 and 53 in place. Forexample, the top cover 51 for the foamed core 43 is shown in plan inFIG. 5 and is in place on the top of the porous foamed metal core 43 inFIG. 4. Similarly, the bottom cover 53 is shown in plan in FIG. 6 and isalso shown in place in FIG. 4.

FIG. 7 shows a preliminary step to applying side covers for heat sink41. The porous foamed metal core is shown at 43 covered only by topcover 51 and bottom cover 53. Feelers 57 and 59 are configured toconform to the profile of the fins, 45, 49, etc. and the spacestherebetween. The feelers are 5,000ths (0.005) inches in thickness andare preferably of light weight aluminum such that they will melt in thebrazing process to fill in the gaps between the side covers 61 (FIG. 9)and 63 (FIG. 11) when the components are assembled and placed in thebrazing oven.

FIG. 12 shows an almost completed product, taken from the brazing oven,with the porous metal foamed core 43 zigzag shape showing in dottedlines as being totally enclosed.

Comparing the outer right hand edge of FIG. 12 to the right cover side63 of FIG. 11, it will be seen that the latter has been milled betweenadjacent pairs of fins so-formed, such as 45 and 49, to become recess46. Now, the heat sink is an integral unit with the exception of thetiny gaps 71 (FIG. 12) adjacent either side of the fins, such as 45 or49. These slots are usually filled with epoxy to complete the sealing ofthe metal foamed core with its fluid path which extends into each fin.

While viewing FIG. 12, it should be stated that the PWB cards 75 of FIG.24 usually will have their metal ground plate 77 clamped in each recess46 by a hexagonally not driven wedge locking device 81 which fills therest of the void or recess 46 and clamps the board firmly against theflanges where the liquid circulation is very closely adjacent, that iscutting down on the L dimension in the formula. This will be covered indetail infra.

Returning to the sheet of drawings including FIGS. 8, 9, 10, 10A and10B, it will be seen that FIG. 8 merely sets forth the prior art, againshown at 27. FIGS. 10A and 10B show the blocks 87 and 89 which terminateeach heat sink 41. Their purpose is two-fold, namely to cover theexposed porous metal foamed core 43, making a fluid tight connectionwith the side covers and top cover, being brazed thereto and providedwith the valve having an opening in the opposite faces of the blocks 87and 89 to control the admission of fluid. Such valves or valve openingsare shown at 91 and 93.

First, in making a heat sink, a rectangular block of the porous metalfoam is obtained from Energy Research and Generation Company of Oakland,California. The foamed linear rectangular block includes the top andbottom covers 51 and 53 but without configurations, i.e. no shaping hasbeen done and again, this is simply purchased as a rectangular elongatedblock having the foam exposed on both sides and at both ends. It isunderstood that the top and bottom cover plates have been brazed to thefoam 43.

The second step in the process is to employ a conventional programmedelectric discharge machine (EDM) simultaneously to cut the irregularslots, e.g., 95 and 97 of FIG. 4 into all three of the top plate 51,porous metal block (not shown) and bottom plate 53. The EDM machine isprogrammed to make these precise cuts continuously while the foamedblock and top and bottom plates are immersed in water. Wires make thetransitions into the cuts but do not actually touch the material, simplyproviding energy for the precise machining necessary to obtain theshapes of FIGS. 4, 5 and 6. This machine has several settings, and ifthe 5,000 mil setting is used, a slow procedure provides a very smoothcut. If the 8,000 mil cutting is used, the cutting is faster but thematerial is not quite so smooth. If the 11,000 mil setting is used, alarge gap is formed and two feelers 57, 59 may be employed on each sideof the foamed metal to fill the large gap encountered between the sidecovers and the foamed block. While all of these setting may be employed,it is typical to use the 8,000 mil setting.

Referring to FIGS. 7 and 11, the core 43 with top plate 51 and bottomplate 53 is placed into a jig and the appropriate number of feelers 57and 59 are pressed into the lateral openings of the core. Then, the sidecovers 61 and 63 are pressed tightly against the feelers and core, andthe two end blocks 87 and 89 are placed against the ends of the foamedmetal block. A spring loaded tool maintains this assembled position ofall the components mentioned while it is put into a vacuum brazing oven(pasteless) and heated to braze the components together, includingmelting the feelers so that they actually disappear. Thereafter, thegaps 71 of FIG. 12 are filled with epoxy and the integral heat sinkunits are finished, ready for assembly into a housing 101 of FIG. 13capable of withstanding heat and pressure and circulating the light oilthroughout the heat sinks 41 to carry away the heat from the printedwiring boards. It is possible to use a conventional box for thispurpose, but it must include the heat sinks 41 of the present invention,as shown in FIG. 14. But, for the present lightweight housing 101, theseheat sinks are affixed to rigid top and bottom plates, such as 103 inFIG. 14, while being spaced apart by the fixtures 105, temporarily.Bellows type conduits 107 are connected between the end blocks, e.g.,89--89', and between end block 87--87', etc. The heat sinks 41 areusually called cold plates, particularly when assembled.

FIG. 14 shows one layer of the heat sink construction, and four or fivelayers are required to accommodate the height of each board.Accordingly, the fluid inlet and outlet manifold connectors 109 and 111each include four short conduits 110 for connection to the respectivelayers.

FIG. 15 shows a completed rack housing having four levels of heat sinks41, stacked in the locations shown in FIG. 14.

FIGS. 16 and 17 show exploded views of the lightweight configuration ofthe PCB rack, which is preferably constructed of composite materials.FIG. 18 shows the lightweight graphite channels.

In these FIGs., the housing top 104 and housing bottom 103 arepreferably porous metal cores 105, best seen in FIG. 15, with aluminumcover plates. They include conventional built-in EMI (electro magneticimpulse) shielding. The footage members 106 and 107' (FIG. 16) arelightweight material and carry the manifolds 108 and 109. The footagemembers 106 and 107' are bonded to the other structure and includeconventional built-in damping. The connector or dummy plate 151 bolts tothe bonded structure.

In FIG. 17, the liquid transfer bellows or tubes 107 (see also FIG. 14)are bonded to the cold plate assembly using LOCTITE RC/680 tube-bondingadhesive. The heat sink (coldplate) 41 and the window frames are bondedtogether using HYSOL and #EA 9395 Epoxy, the window frames being betweenthe stacked heat sinks (cold plates) 41 (FIGS. 18 and 19).

In FIG. 18, the graphite frame 131 is shown with a top flange 133 and abottom flange 135. It includes the four window frame channels 137, 139,141, and 143 with cross frame members 147 to correspond to theoppositely disposed four frames channels 137', 139', 141' and 143'. Itis HYSOL bonded together. The purpose of the graphite unit is to fitwithin the housing 101 of FIG. 13 to receive and hold the heat sinks 41.This is better seen in FIG. 19, wherein two heat sinks 41 are shown incross-section with epoxy bonding material 161 and 163 sandwiching alayer of fiberglass 164 supported by the graphite channel 139 with thesimilar structure of epoxy 165 and 167 sandwiching fiberglass layer 169below the graphite in connection with the lower heat sink 41. It hasbeen found that the fiberglass 164 and 169 is really not essential andthe cost can be reduced somewhat by removing layers 164 and 169.

Finally, FIGS. 20 through 24 show the preferred way of clamping theboards against the thin-walled fins, as shown in FIG. 24 to ensure lowresistance heat transfer out of the boards as quickly as possible to themoving fluid. The wedge locking device 81 simply includes an elongatedscrew 171 extending through five pieces of tubing 173, 174, 175, 176,and 177. These five pieces of tubing are cut on 45 degree angles so thatwhen the hexagonal nut 181 is turned, as thus shown in FIG. 23, the twopieces 174 and 176 are wedged in one direction relative to the pieces173, 175, and 177 to ensure tightening of the board ground member 77(FIG. 24) against the walls 191 and 193 of the recess 195 betweenadjacent fins 149 and 145 of FIG. 24. Of course, the same would be truein FIG. 12 with respect to the recess 46. The board would be clamped onthe upper side of rib 45 and the next board would be clamped on theupper side of rib 49 so that the entire board could leak heat to singleribs on each end.

By way of example, for a 5.5 inch by 5.95 inch card, R₁ of FIG. 24 is0.050 inches thick (wall 193) and wall 191 (T₁) is 0.065 inches thickfor the fins 145 and 149.

In keeping with the lightweight construction, the preferred material forthe many devices 81 is aluminum whenever a lightweight composite is notused.

What is claimed is:
 1. A heat sink for use in a fluid cooling systemcomprising, in combination:a lightweight metal conduit having hollowradiating fins; a fluid porous metal foamed core extending in zigzagfashion within and axially of the conduit to cause fluid entering saidcore at one end to follow tortuous path through the conduit via saidcore to its other end; and, means for introducing said fluid to saidpath.
 2. The heat sink of claim 1, wherein:said conduit and fins arecomprised of zigzag configured foamed metal having a conforming top andbottom cover of thin aluminum, and zigzag conforming side covers.
 3. Aintegrated lightweight printed circuit board rack and housing,comprising in combination:a plurality of heat sinks, each comprising; alightweight metal conduit having hollow radiating fins disposedlaterally of the conduit; a fluid porous metal foamed core extending inzigzag fashion within the conduit and fins and axially of the conduit tocause fluid entering the core at one end to follow a tortuous paththrough the conduit via said core to its other end; said heat sinksincluding interconnecting blocks on each end of the conduits for fluidflow; said housing for the rack including a rigid bottom; several ofsaid heat sinks arrayed on the bottom in spaced apart relationthereacross with said interconnecting blocks on first ends of saidconduits of said heat sinks being adjacent to one edge of said bottomand said interconnecting blocks on ends of said conduits opposite saidfirst ends being adjacent to an edge of said bottom opposite to said oneedge; fluid transfer bellows connected between said interconnectingblocks adjacent said one edge and further fluid transfer bellowsconnected between said interconnecting blocks adjacent said oppositeend; said interconnected heat sinks and fluid transfer bellowscomprising a first heat sink layer; graphite window frame means; aplurality of further heat sink layers respectively stacked on said firstlayer and all layers received in and supported by said window framemeans within said housing; a fluid input manifold in fluid connectionwith one interconnecting block in each layer and an output manifoldconnected to a different connecting block in each layer; and, a top andsides for said housing enclosing all of said heat sinks; means forintroducing fluid to said input manifold and extracting fluid from saidoutput manifold whereby printed circuit boards adjacent said finsreadily give up their heat to the fluid.
 4. The rack and housing ofclaim 3, wherein:said top and bottom of said housing comprise cores ofporous foamed metal.
 5. A heat sink for use in a cooling system,comprising in combination:a substantially rectangular metal conduithaving hollow radiating fins extending outwardly from a pair of oppositesides of the conduit; said fins being substantially uniformly spacedalong said conduit with fins on one side being staggered relative tofins on the other side; and, a fluid porous metal foamed coreconsecutively extending in said conduit first into a fin on said oneside and thence into a fin on said other side for the length of theconduit.